Die for shaped crystal growth from a molten bath

ABSTRACT

A die for drawing crystals from a molten bath, has a body bored with at least one longitudinal capillary channel arranged between a lower face of the body. The die is intended to be immersed in the molten bath and an upper face of the body intended to support the liquid coming from the molten bath through the capillary channel. The die upper face has a flat surface onto which the capillary channel emerges, and this flat surface forms an angle θ less than 90° with the longitudinal axis of the capillary channel.

TECHNICAL FIELD

The subject of this invention is a die for the drawing of crystals froma molten bath, that may be used in particular, in solidification methodsreferred to as capillary growth methods.

These methods consist of fabricating an item made of a polycrystallineor monocrystalline material by drawing it through a die from a bath ofthe molten material and crystallizing the molten material coming out ofthe die on a seed.

The invention is particularly applicable to the production of items witha complex shape, made of various materials, for example, oxides,silicon, aluminum, or other metals, in a mono- or poly-crystalline form.

STATE OF THE PRIOR TECHNOLOGY

Methods of manufacturing items made of a monocrystalline orpolycrystalline material by growth from a molten bath have been knownfor a long time and have been described, for example, in document 1: H.E. La Belle, Jr., Journal of Crystal Growth, 50, 1980, pages 8 to 17,and in document 2: Y. A. Tatarchenko, Shaped Crystal Growth, KluwerAcademic Publishers, 1993, 6, pages 1 to 18.

In document 1, the drawing method referred to as EFG (Edge defined,Film-fed Growth) is especially described, a method wherein a die is usedthat comprises a capillary channel produced in a material that is wettedby the molten bath.

In document 2, various crystal growth methods are described, and inparticular, preforming methods that use dies of suitable shape.

Document 3: EP-A-653 504 describes a method of manufacturing items in apolycrystalline or monocrystalline material by crystal growth from aliquid bath of the molten material. In this method, a die is usedthrough which the molten material that one wishes to crystallize on theseed is drawn, and the die and/or the seed is subjected to at least onetranslational movement in a plane perpendicular to the direction ofdrawing. In particular, this allows the production of items of complexshape.

Document 4: U.S. Pat. No. 4,430,305 describes a die with capillary tubessuitable for drawing items made of silicon having the shape of a plate.This die has a particular shape in order to encourage the appearance ofthe impurity SiC only on one side of the plate being produced.

Recent developments in preforming methods make use of drawing deviceswhich permit the development of particular shapes by the use of verticaldrawing, combined with relative horizontal movement of the seed on whichsolidification is occurring, with respect to the die which is supplyingthe molten material.

Certain installations are derived from a Czochralski furnace, where aseed and crucible are driven in a rotational movement ω about a verticalaxis Z, and a translational movement along this same axis.

FIG. 1 appended shows the simple case of traditional preforming of abar, where only vertical translation of the seed is used.

In such a method, supply takes place in general from a crucible 1containing the liquid 2 to be solidified, through a die 3 whichcomprises at least one internal bore, either a capillary channel 5,wherein the liquid coming from the crucible can circulate, or bycapillary rise in the case where the liquid wets the die material, orunder the effect of a pressure difference between the surface of theliquid 2 in the crucible and the outlet from the die. At the outlet fromthe die 3, a liquid meniscus 7 is obtained and the solidification of theliquid is initiated on a monocrystalline seed 9 and is continued by thedrawing of this seed and of the formed solid 10, upwards in thedirection of the arrow, at speed Vz. The precise shape of the solidobtained depends on the distance between the solidification interface 11and the outlet from the die 3, but always stays close to the latter. Theposition of this interface can be adjusted by controlling the supply ofheat (calories) to the crucible 1.

For local preforming, there is an extra movement: the rotation of theseed about itself.

In document 3, the method described enables one to draw curved shapes,using traditional preforming (i.e. without rotating the seed), or rounditems using local preforming (i.e. with rotation of the seed).

FIG. 2 appended shows how to obtain a curved rod 19 of circularcross-section, by using the combined movements of vertical drawing andtranslation in the horizontal direction, of both the seed 9 and the itemundergoing solidification.

In this figure, one can see the die 3 bored with a central channel 5,through which the liquid moves to form a meniscus 7 at the outlet of thedie with a solid-liquid interface 11 of the item undergoingsolidification 19. In this case, the seed 9 is displaced in the verticaldirection at speed Vz causing a horizontal translational movement to bedescribed at the die at a suitable speed Ux. The diameter of the die ismatched to provide the desired diameter d of the curved rod 19, thisdiameter also depending on the position of the interface 11 above thedie 3. In order to obtain a rod having the shape of an elliptical armwith half-axes a and b, the horizontal translational movement of the dieis carried out between a first position wherein the seed and the die arealigned and a second position wherein the seed is the distance b fromthe die, and the speed of vertical drawing is regulated and thehorizontal translation function Ux of the die in relation to time t, issuch that one has:

Vz=(πa/2τ)·sin(πt/2τ) and Ux=(πb/2τ)·cos(πt/2τ)

with τ representing the duration of drawing and a and b representingrespectively the half-axes of the ellipse.

In the methods described above one uses the same dies as those developedfor traditional preforming, that is to say dies whose upper face ishorizontal, or perpendicular to the axis of the bore and to thedirection of drawing. With such dies, the control of the shape given tothe preforming material occurs only at the fluid meniscus 7. Itsgeometry depends on the capillary constant of the molten material andthe contact limit conditions on the die. The shape of the meniscus andits contact on the die are therefore the only parameters which locallydefine the shape of the crystal.

Whatever the height Hm of the meniscus, linked to the nature of thematerial and to the thermal profile in the furnace, the equilibrium ofthe fluid drop on the horizontal die always leads, for large drawingangles, to too small a crystal thickness for the method of pulling awayhorizontally to be viable.

It has also been observed that the use of dies wherein the upper face ishorizontal is also restricted by the problem of attachment of themeniscus over the entire section of this face. The control of the shapethen becomes deficient and makes the method of pulling away horizontallynon-viable.

In FIG. 3, which represents diagrammatically a preforming methodanalogous to that in FIG. 2, and which uses the same reference numbers,it can be seen that the thickness Ec of the crystal 19 drawn with thedie 3 with a horizontal upper face, depends on the characteristicdimension Ef of the die 3, the height of the meniscus Hm and the drawingangle α. This drawing angle is uniquely a function of the speeds imposedby the growth. For example, in FIG. 2, where the speed of horizontaltranslation is designated Ux and the speed of vertical drawing is Vz,the drawing angle is given by: $\begin{matrix}{{{tg}{\quad \quad}\alpha} = \frac{Ux}{Vz}} & \text{(R1)}\end{matrix}$

When the height of the meniscus Hm is negligible compared with Ef (thisis the case for example for sapphire), the thickness Ec of the crystalis given by:

Ec=Ef·cos α  (R2)

The equation (R2) reveals a major handicap of preforming usingtraditional dies: the geometric limitation of the drawing angle requiredto keep the crystal from having zero thickness. Quite obviously, adrawing angle of 90° to the vertical is impossible with a traditionaldie with a horizontal top.

Hence, these traditional dies are not suitable for large drawing anglesand, in particular to create horizontal pulling away of the crystalgrowth.

DESCRIPTION OF THE INVENTION

The precise subject of this invention is a die for drawing mono-crystalsor poly-crystals from a molten bath, which remedies this disadvantage.

For this purpose, the invention proposes a die for the drawing ofmono-crystals or poly-crystals from a molten bath, comprising a bodybored with at least one longitudinal capillary channel arranged betweena lower face of the body intended to be immersed in the molten bath andan upper face of the body intended to support the liquid coming from themolten bath through the capillary channel wherein said upper facecomprises at least one flat surface at which the capillary channel orchannels emerge, said flat surface forming an angle θ less than 90°, forexample 45°, with the longitudinal axis of the capillary channel orchannels.

This die therefore comprises an upper flat surface supporting the liquidundergoing solidification, which is inclined with respect to thehorizontal, which enables one to produce, under satisfactory conditions,the horizontal growth necessary for the development of certain shapes,such as, for example, a round ellipsoid or paraboloid, items bentthrough 90° etc.

According to the invention, this flat surface at which the capillarychannel emerges thereby defines a single unique plane in space.

The die of the invention may comprise a single capillary channel, forexample of circular, square or rectangular cross section, or in theshape of a circular crown. It may also comprise several capillarychannels of circular section or even several capillary channels ofcircular section emerging below the upper face of the die into a singlecapillary channel of rectangular cross section, as described in document4: U.S. Pat. No. 4,430,305.

The edges of the opening of the capillary channel on the flat surface ofthe upper face of the die can make an angle of 90° with this flatsurface, or form an angle less than 90°, for example 45°, with thissurface.

According to a preferred embodiment, the die of the invention comprisesa single capillary channel and an upper face, only a part of which formsthe flat surface making an angle of less than 90° with the longitudinalaxis of the capillary channel, the capillary channel emerging onto theflat surface through a section inclined with respect to the longitudinalaxis of the channel.

This section generally has a smaller cross section than that of the restof the capillary channel.

Furthermore, it is preferable that the edges of the opening throughwhich the section emerges onto the flat surface of the upper face of thedie form an angle less than 90° with this flat surface.

By way of example, this angle may be 45° like angle θ.

The dies of the invention may be produced in various materials oncondition that the material is chemically inert to the molten bath andthat it is preferably wetted by the molten bath. They may, inparticular, be produced in molybdenum.

Other characteristics and advantages of the invention will better becomeapparent on reading the description which follows of embodiment examplesgiven, it is understood, as illustrative and non-limitative examples,and which make reference to the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1, already described, diagrammatically illustrates the traditionalmethod of drawing of the prior art, wherein, only the seed is moved in avertical translational movement.

FIGS. 2 and 3, already described, illustrate the traditional preformingmethod of the prior art using two translational movements.

FIG. 4 is a diagrammatic representation of a method of preforming acurved shape conforming to the method described in EP-A-653 504, butusing a die conforming to the invention.

FIG. 5 is a perspective view of a die conforming to the invention.

FIGS. 6A, 6B, 6C illustrate the production of a hemi-spherical crownusing a die conforming to the invention.

FIGS. 7A, 7B, 7C illustrate the production of an elbow-shaped item witha die conforming to the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

Referring to FIG. 4, wherein the same reference numbers have been usedas in FIGS. 1 to 3 to refer to the die 3, the seed 9, the crystal 19,the fluid meniscus 7 and the solidification interface 11, it can be seenthat, using a die conforming to the invention, the upper face 6 of thisdie makes an angle θ less than 90° with the longitudinal axis of thecapillary channel 5, or with the vertical.

Under these conditions, the characteristic dimension Ec of the crystalproduced is always linked to that of the die and to the drawing angle α.The height Hm of the meniscus is also involved, but the shape of thecrystal obeys the simple equation (R3) below, whatever the material,when Hm is negligible compared with the dimension Ef of the die.$\begin{matrix}{{Ec} = {{{Ef}\left( {1 - \frac{\sin \quad \alpha}{{tg}\quad \theta}} \right)}.}} & \text{(R3)}\end{matrix}$

Simple geometric considerations show that the range of drawing angles αthat are theoretically possible, passes from [−π/2; π/2] with thehorizontal die (equation R1) to [−θ; π/2] with the die of the invention(equation R3).

The range of possible values for α is reduced (π to π/2+θ) but throughthe inclination angle θ, a translation equivalent to the interval thatis theoretically possible is provided, which is of particular interestwhen the method only counts on values with a constant sign for thehorizontal speed. This is commonly done for the production of rounditems of variable cross section. The inversion of Ux remains possiblewithin the limit where the drawing angle α remains greater than −θ. Inthis latter case, a die can be used that, on its upper face, comprisestwo flat surfaces that respectively make angles of +θ and −θ with theaxis of the capillary channel. In this case, each flat surface has anopening at which the capillary channel emerges. Two capillary channelsmay also be used, each associated with a single opening. Such a diepermits a range of drawing angles that is completely symmetrical.

With the die of the invention, there is a temperature difference in thecrystallization zone. In effect, the height difference between the highHf and low Bf points of the flat surface 6 of the die leads to atemperature difference between these same points in the axiallysymmetric hot zone of the furnace. Bf is at a higher temperature thanHf. Against all expectations, this thermal gradient improves growthwhich facilitates the attachment of the meniscus 7 at Hf. and guaranteesexcellent control of the shape at this point. Furthermore, varioussolutions such as thermal screening and the die support geometry,enables one to resolve the problem of overheating at Bf and to providecontrol of the method at this point that is just as good as at Hf.

Hence, the die of the invention finally resolves the problem of thehorizontal growth that is necessary to create certain shapes such as around ellipsoid or paraboloid and items with a 90° bend.

Furthermore, the invention may also be used in all traditional and localpreforming methods such as those described in document 3: EP-A-653 504,thereby offering the possibility of changing the shape of the item beingdrawn in a controlled and continuous manner.

Finally, the use of this die, whatever the material being drawn, permitstrue control of the method based on the geometric specifications of theitem being produced.

Fine thermal control enables one to always remain within thehypothetical conditions where the height of the meniscus is very smallcompared with the diameter of the die Ef and hence to make possible theuse of equations R1 and R3 given above that relate to the drawing angleα and the thickness Ec of the drawn crystal.

In FIG. 5, a die is shown that conforms to a preferred embodiment of theinvention. This die comprises a body 51, a lower face 53, an upper face55 and a shoulder 57 used as a support in the drawing installation. Thisdie is bored with a capillary channel 59 of rectangular section, whichemerges at one end, at the lower face 53 and at the other end at theupper face 55 through the opening 61 that is also of rectangularsection.

This upper face is so conformed that it presents an inclined flatsurface 63 that makes an angle θ less than 90°, for example 45°, withthe longitudinal axis of the capillary channel 59, the remainder of theupper face 55 making an angle of approximately 90° with the longitudinalaxis of the capillary channel. At the surface 63, the capillary channel59 emerges into the opening 61 through an inclined section 65 of smallersection the edges of which broaden out since they make an angle lessthan 90°, for example 45°, with the surface 63.

This die is particularly suitable for the preforming of sapphire(Al₂O₃). Its,.shape and that of the base which supports it have beenespecially researched in order to optimize the thermal profile on theinclined surface 63. The meniscus which is formed on this surfacethereby preserves excellent contact limit conditions throughout thegrowth.

This die is preferably produced in molybdenum.

In FIGS. 6A to 6C, the production of a hemi-spherical crown is shownusing the die of the invention having an angle θ of 45°. In thesefigures, the same reference numbers have been used as in FIG. 2.

In this case, the local preforming method is used, this methodconsisting of varying the drawing angle α in a continuous fashion,through the combination of a vertical drawing movement at speed Vz, ahorizontal translational movement at speed Ux and a rotational movementof the seed 9 at speed ω.

At the beginning, FIG. 6A, a vertical section is produced by verticaldrawing and simultaneous rotation of the seed 9. Next, thehemi-spherical part is formed by varying the growth parameters (Vz, Uxand ω) in order to generate the sphere. In FIG. 6B, the drawing takesplace practically horizontally, then the drawing angle changes andreduces (FIG. 6C) as the hemi-spherical crown is formed.

In FIGS. 7A to 7C, the same reference numbers have been used and theproduction of an elbow piece has been shown using traditionalpreforming, without rotation of the seed, with the die of the invention.

At the start (FIG. 7A), a vertical section is produced by simple drawingwith a single movement of the seed in the vertical direction at speedVz.

Next the elbow part is formed (FIG. 7B) by simultaneously subjecting theseed to a horizontal and a vertical movement at speeds Ux and Vz.

After producing the elbow part, growth is continued (FIG. 7C) only usingthe vertical movement Vz as in FIG. 7A.

Of course, one may combine the methods described relating to FIGS. 6A,6B, 6C, 7A, 7B and 7C to form a hemispherical crown and finish it with abar bent at right angles by stopping the rotational movement of theseed.

Therefore, the invention offers numerous advantages since it permits theproduction of complex shapes with materials that are difficult tomachine, such as, for example, sapphire which has optical, thermal,mechanical and chemical properties that are very interesting.

The fields of application for this material extend from jewelry (watchglasses), to the production of ultra-strong technological components(nozzles and high pressure windows, cutting tools), and to militarydevices for infra-red transmission (IR domes). Bearing in mind theexceptional hardness of sapphire, preforming substantially reduces itsproduction costs.

The device for growth by inclination of the solidification interfaceextends preforming to the production of components that until now havebeen impossible to produce with traditional dies. The hemisphericalcrown, cladding with particular shapes (for example, hollow elbowtubes), and the special crucibles are applications that may be developedthrough this method.

Other materials, even those that are much more machinable than sapphiremay also be used with this method, since certain shapes obtained bypreforming are totally impossible to produce by machining (typicallyhollow items and items bent at right angles).

References quoted

1. La Belle Jr., J. of Crystal Growth, 50, 1980, p. 8-17,

2. Tatarchenko, Shaped Crystal Growth, Kluwer Academic Publishers, 1993,p. 1∫18,

3. EP-A-653 504

4. U.S. Pat. No. 4,430,305

What is claimed is:
 1. Die for drawing mono-crystals or poly-crystals from a molten bath, comprising a body bored with at least one longitudinal capillary channel arranged between a lower face of the body intended to be immersed in the molten bath and an upper face of the body intended to support liquid coming from the molten bath through the capillary channel or channels, wherein said upper face comprises a planar flat surface onto which the capillary channel or channels emerge, said flat surface forming an angle θ less than 90° with the longitudinal axis of the capillary channel or channels.
 2. Die according to claim 1, comprising a single capillary channel of circular, square or rectangular section.
 3. Die according to claim 1, comprising a single capillary channel, the cross section of which is a circular crown.
 4. Die according to claim 1, comprising several capillary channels that emerge below the upper face into a single capillary channel of rectangular section.
 5. Die according to claim 1, characterized in that the angle θ is equal to 45°.
 6. Die according to claim 1, wherein edges of the opening of the capillary channel on the flat surface of the upper face of the die form an angle less than 90° with this flat surface.
 7. Die according to claim 1, comprising a single capillary channel and an upper face, only one part of which forms the flat surface making an angle θ less than 90° with the longitudinal axis of the capillary channel, the capillary channel emerging onto this flat surface through a section inclined with respect to the longitudinal axis of the channel.
 8. Die according to claim 7, wherein the section has a cross section smaller than that of the rest of the capillary channel.
 9. Die according to claim 8 wherein edges of the opening through which the section emerges onto the flat surface of the upper face of the die form an angle less than 90° with this flat surface.
 10. Die according to claim 1, wherein edges of the opening of the capillary channel make an angle of 45° with the flat surfaces of the upper face of the die.
 11. Die according to claim 1, produced in molybdenum.
 12. Method of manufacturing an item made of a polycrystalline or monocrystalline material, wherein the item is formed from a bath of molten material by drawing molten material through a die bored with at least one capillary channel and crystallizing the molten material coming out of the capillary channel in contact with a seed, the seed and/or the die being subjected to at least one translational movement and/or rotational movement in a plane perpendicular to the direction of drawing, wherein a crystallization interface of the molten material forms an angle θ less than 90° with the direction of drawing. 